@c ***************************************************************************
@node Philosophy
@chapter Philosophy

The foremost goal of the GNUnet project is to become a widely used,
reliable, open, non-discriminating, egalitarian, unfettered and
censorship-resistant system of free information exchange.
We value free speech above state secrets, law-enforcement or
intellectual property. GNUnet is supposed to be an anarchistic network,
where the only limitation for peers is that they must contribute enough
back to the network such that their resource consumption does not have
a significant impact on other users. GNUnet should be more than just
another file-sharing network. The plan is to offer many other services
and in particular to serve as a development platform for the next
generation of decentralized Internet protocols.

@menu
* Design Goals::
* Security & Privacy::
* Versatility::
* Practicality::
* Key Concepts::
@end menu


@c ***************************************************************************
@node Design Goals
@section Design Goals

These are the core GNUnet design goals, in order of relative importance:

@itemize
@item GNUnet must be implemented as free software.
@item GNUnet must only disclose the minimal amount of information necessary.
@item GNUnet must be decentralised and survive Byzantine failures in any position in the network.
@item GNUnet must make it explicit to the user which entities must be trustworthy when establishing secured communications.
@item GNUnet must use compartmentalization to protect sensitive information.
@item GNUnet must be open and permit new peers to join.
@item GNUnet must be self-organizing and not depend on administrators.
@item GNUnet must support a diverse range of applications and devices.
@item The GNUnet architecture must be cost effective.
@item GNUnet must provide incentives for peers to contribute more resources than they consume.
@end itemize


@node Security & Privacy
@section Security & Privacy

GNUnet's primary design goals are to protect the privacy of its users and to
guard itself against attacks or abuse. GNUnet does not have any mechanisms
to control, track or censor users. Instead, the GNUnet protocols aim to make
it as hard as possible to find out what is happening on the network or to
disrupt operations. 

@node Versatility
@section Versatility

We call GNUnet a peer-to-peer framework because we want to support many
different forms of peer-to-peer applications. GNUnet uses a plugin
architecture to make the system extensible and to encourage code reuse.
While the first versions of the system only supported anonymous file-sharing,
other applications are being worked on and more will hopefully follow in the
future. A powerful synergy regarding anonymity services is created by a large
community utilizing many diverse applications over the same software
infrastructure. The reason is that link encryption hides the specifics
of the traffic for non-participating observers. This way, anonymity can
get stronger with additional (GNUnet) traffic, even if the additional
traffic is not related to anonymous communication. Increasing anonymity is
the primary reason why GNUnet is developed to become a peer-to-peer
framework where many applications share the lower layers of an increasingly
complex protocol stack. If merging traffic to hinder traffic analysis was
not important, we could have just developed a dozen stand-alone applications
and a few shared libraries. 

@node Practicality
@section Practicality

GNUnet allows participants to trade various amounts of security in exchange
for increased efficiency. However, it is not possible for any user's security
and efficiency requirements to compromise the security and efficiency of
any other user. 

For GNUnet, efficiency is not paramount. If there is a more secure and still
practical approach, we would choose to take the more secure alternative.
@command{telnet} is more efficient than @command{ssh}, yet it is obsolete.
Hardware gets faster, and code can be optimized. Fixing security issues as
an afterthought is much harder. 

While security is paramount, practicability is still a requirement. The most
secure system is always the one that nobody can use. Similarly, any
anonymous system that is extremely inefficient will only find few users.
However, good anonymity requires a large and diverse user base. Since
individual security requirements may vary, the only good solution here is to
allow individuals to trade-off security and efficiency. The primary challenge
in allowing this is to ensure that the economic incentives work properly.
In particular, this means that it must be impossible for a user to gain
security at the expense of other users. Many designs (e.g. anonymity via
broadcast) fail to give users an incentive to choose a less secure but more
efficient mode of operation. GNUnet should avoid where ever possible to
rely on protocols that will only work if the participants are benevolent.
While some designs have had widespread success while relying on parties
to observe a protocol that may be sub-optimal for the individuals (e.g.
TCP Nagle), a protocol that ensures that individual goals never conflict
with the goals of the group is always preferable.

@node Key Concepts
@section Key Concepts

In this section, the fundamental concepts of GNUnet are explained.  Most of
them are also described in our research papers.  First, some of the concepts
used in the GNUnet framework are detailed.  The second part describes concepts
specific to anonymous file-sharing.

@menu
* Authentication::
* Accounting to Encourage Resource Sharing::
* Confidentiality::
* Anonymity::
* Deniability::                       
* Peer Identities::
* Zones in the GNU Name System (GNS Zones)::
* Egos::
@end menu

@node Authentication
@subsection Authentication

Almost all peer-to-peer communications in GNUnet are between mutually
authenticated peers. The authentication works by using ECDHE, that is a
DH key exchange using ephemeral eliptic curve cryptography. The ephemeral
ECC keys are signed using ECDSA. The shared secret from ECDHE is used to
create a pair of session keys (using HKDF) which are then used to encrypt
the communication between the two peers using both 256-bit AES and 256-bit
Twofish (with independently derived secret keys). As only the two
participating hosts know the shared secret, this authenticates each packet
without requiring signatures each time. GNUnet uses SHA-512 hash codes to
verify the integrity of messages. 

In GNUnet, the identity of a host is its public key. For that reason,
man-in-the-middle attacks will not break the authentication or accounting
goals. Essentially, for GNUnet, the IP of the host has nothing to do with
the identity of the host. As the public key is the only thing that truly
matters, faking an IP, a port or any other property of the underlying
transport protocol is irrelevant. In fact, GNUnet peers can use
multiple IPs (IPv4 and IPv6) on multiple ports --- or even not use the
IP protocol at all (by running directly on layer 2). 

GNUnet uses a special type of message to communicate a binding between
public (ECC) keys to their current network address. These messages are
commonly called HELLOs or peer advertisements. They contain the public key
of the peer and its current network addresses for various transport services.
A transport service is a special kind of shared library that
provides (possibly unreliable, out-of-order) message delivery between peers.
For the UDP and TCP transport services, a network address is an IP and a port.
GNUnet can also use other transports (HTTP, HTTPS, WLAN, etc.) which use
various other forms of addresses. Note that any node can have many different
active transport services at the same time, and each of these can have a
different addresses. Binding messages expire after at most a week (the
timeout can be shorter if the user configures the node appropriately). This
expiration ensures that the network will eventually get rid of outdated
advertisements.@
More details can be found in the paper @uref{https://gnunet.org/transports, A Transport Layer Abstraction for Peer-to-Peer Networks}.

@node Accounting to Encourage Resource Sharing
@subsection Accounting to Encourage Resource Sharing

Most distributed P2P networks suffer from a lack of defenses or precautions
against attacks in the form of freeloading. While the intentions of an
attacker and a freeloader are different, their effect on the network is the
same; they both render it useless. Most simple attacks on networks such as
Gnutella involve flooding the network with traffic, particularly with
queries that are, in the worst case, multiplied by the network. 

In order to ensure that freeloaders or attackers have a minimal impact on the
network, GNUnet's file-sharing implementation tries to distinguish
good (contributing) nodes from malicious (freeloading) nodes. In GNUnet,
every file-sharing node keeps track of the behavior of every other node it
has been in contact with. Many requests (depending on the application) are
transmitted with a priority (or importance) level. That priority is used to
establish how important the sender believes this request is. If a peer
responds to an important request, the recipient will increase its trust in the
responder: the responder contributed resources. If a peer is too busy to
answer all requests, it needs to prioritize. For that, peers to not take the
priorities of the requests received at face value. First, they check how much
they trust the sender, and depending on that amount of trust they assign the
request a (possibly lower) effective priority. Then, they drop the requests
with the lowest effective priority to satisfy their resource constraints. This
way, GNUnet's economic model ensures that nodes that are not currently
considered to have a surplus in contributions will not be served if the
network load is high. More details can be found in @uref{https://gnunet.org/ebe, this paper}.

@node Confidentiality
@subsection Confidentiality

Adversaries outside of GNUnet are not supposed to know what kind of actions a
peer is involved in. Only the specific neighbor of a peer that is the
corresponding sender or recipient of a message may know its contents, and even
then application protocols may place further restrictions on that knowledge.
In order to ensure confidentiality, GNUnet uses link encryption, that is each
message exchanged between two peers is encrypted using a pair of keys only
known to these two peers. Encrypting traffic like this makes any kind of
traffic analysis much harder. Naturally, for some applications, it may still
be desirable if even neighbors cannot determine the concrete contents of a
message. In GNUnet, this problem is addressed by the specific
application-level protocols (see for example, deniability and anonymity in
anonymous file sharing).

@node Anonymity
@subsection Anonymity

@menu
* How file-sharing achieves Anonymity::
@end menu

Providing anonymity for users is the central goal for the anonymous
file-sharing application. Many other design decisions follow in the footsteps
of this requirement. Anonymity is never absolute. While there are various
@uref{https://gnunet.org/anonymity_metric, scientific metrics} that can help quantify the level of anonymity that a
given mechanism provides, there is no such thing as complete anonymity.
GNUnet's file-sharing implementation allows users to select for each
operation (publish, search, download) the desired level of anonymity.
The metric used is the amount of cover traffic available to hide the request.
While this metric is not as good as, for example, the theoretical metric
given in @uref{https://gnunet.org/anonymity_metric, scientific metrics}, it is probably the best metric available to
a peer with a purely local view of the world that does not rely on unreliable
external information. The default anonymity level is 1, which uses anonymous
routing but imposes no minimal requirements on cover traffic. It is possible
to forego anonymity when this is not required. The anonymity level of 0
allows GNUnet to use more efficient, non-anonymous routing.

@node How file-sharing achieves Anonymity
@subsubsection How file-sharing achieves Anonymity

Contrary to other designs, we do not believe that users achieve strong
anonymity just because their requests are obfuscated by a couple of
indirections. This is not sufficient if the adversary uses traffic analysis.
The threat model used for anonymous file sharing in GNUnet assumes that the
adversary is quite powerful. In particular, we assume that the adversary can
see all the traffic on the Internet. And while we assume that the adversary
can not break our encryption, we assume that the adversary has many
participating nodes in the network and that it can thus see many of the
node-to-node interactions since it controls some of the nodes. 

The system tries to achieve anonymity based on the idea that users can be
anonymous if they can hide their actions in the traffic created by other users.
Hiding actions in the traffic of other users requires participating in the
traffic, bringing back the traditional technique of using indirection and
source rewriting. Source rewriting is required to gain anonymity since
otherwise an adversary could tell if a message originated from a host by
looking at the source address. If all packets look like they originate from
a node, the adversary can not tell which ones originate from that node and
which ones were routed. Note that in this mindset, any node can decide to
break the source-rewriting paradigm without violating the protocol, as this
only reduces the amount of traffic that a node can hide its own traffic in. 

If we want to hide our actions in the traffic of other nodes, we must make
our traffic indistinguishable from the traffic that we route for others. As
our queries must have us as the receiver of the reply (otherwise they would
be useless), we must put ourselves as the receiver of replies that actually
go to other hosts; in other words, we must indirect replies. Unlike other
systems, in anonymous file-sharing as implemented on top of GNUnet we do not
have to indirect the replies if we don't think we need more traffic to hide
our own actions.@

This increases the efficiency of the network as we can indirect less under
higher load. More details can be found in @uref{https://gnunet.org/gap, this paper}. 

@node Deniability
@subsection Deniability

Even if the user that downloads data and the server that provides data are
anonymous, the intermediaries may still be targets. In particular, if the
intermediaries can find out which queries or which content they are
processing, a strong adversary could try to force them to censor
certain materials. 

With the file-encoding used by GNUnet's anonymous file-sharing, this problem
does not arise.  The reason is that queries and replies are transmitted in
an encrypted format such that intermediaries cannot tell what the query
is for or what the content is about.  Mind that this is not the same
encryption as the link-encryption between the nodes.  GNUnet has
encryption on the network layer (link encryption, confidentiality,
authentication) and again on the application layer (provided
by @command{gnunet-publish}, @command{gnunet-download}, @command{gnunet-search}
and @command{gnunet-gtk}).  More details can be found @uref{https://gnunet.org/encoding, here}.

@node Peer Identities
@subsection Peer Identities

Peer identities are used to identify peers in the network and are unique for
each peer.  The identity for a peer is simply its public key, which is
generated along with a private key the peer is started for the first time.
While the identity is binary data, it is often expressed as ASCII string.
For example, the following is a peer identity as you might see it in
various places:@
@code{@
 UAT1S6PMPITLBKSJ2DGV341JI6KF7B66AC4JVCN9811NNEGQLUN0@
}

You can find your peer identity by running@
@command{gnunet-peerinfo -s}

@node Zones in the GNU Name System (GNS Zones)
@subsection Zones in the GNU Name System (GNS Zones)

GNS zones are similar to those of DNS zones, but instead of a hierarchy of
authorities to governing their use, GNS zones are controlled by a private key.
When you create a record in a DNS zone, that information stored in your
nameserver.  Anyone trying to resolve your domain then gets pointed (hopefully)
by the centralised authority to your nameserver.  Whereas GNS, being
decentralised by design, stores that information in DHT.  The validity of the
records is assured cryptographically, by signing them with the private key of
the respective zone.

Anyone trying to resolve records in a zone your domain can then verify the
signature on the records they get from the DHT and be assured that they are
indeed from the respective zone.  To make this work, there is a 1:1
correspondence between zones and their public-private key pairs.  So when we
talk about the owner of a GNS zone, that's really the owner of the private
key.  And a user accessing a zone needs to somehow specify the corresponding
public key first.

@node Egos
@subsection Egos

Egos are your "identities" in GNUnet.  Any user can assume multiple
identities, for example to separate his activities online.  Egos can
correspond to pseudonyms or real-world identities.  Technically, an
ego is first of all a public-private key pair.

